Print private/public key

Question

For testing purposes, I want to print my public and private key which are generated on the Arduino via elliptic curve.

The relevant code looks as follows,

#include <Ed25519.h>

void setup()
{
    Serial.begin(9600); // Debugging only
    if (!driver.init())
         Serial.println("init failed");

    uint8_t privateKey[32];
    uint8_t publicKey[32];
    Ed25519::generatePrivateKey(privateKey);
    Ed25519::derivePublicKey(publicKey, privateKey);
    Serial.println(publicKey);
    Serial.print(",");
    Serial.write(privateKey);
}

However, it gives me the following error:

/tmp/arduino_modified_sketch_961866/ask_receiver.pde: In function 'void setup()':
ask_receiver:52: error: call of overloaded 'println(uint8_t [32])' is ambiguous
     Serial.println(publicKey);
                             ^
/tmp/arduino_modified_sketch_961866/ask_receiver.pde:5

2:29: note: candidates are:
In file included from /ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Stream.h:26:0,
                 from /ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:29,
                 from /ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Arduino.h:232,
                 from sketch/ask_receiver.pde.cpp:1:
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:78:12: note: size_t Print::println(const String&) <near match>
     size_t println(const String &s);
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:78:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'const String&'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:79:12: note: size_t Print::println(const char*) <near match>
     size_t println(const char[]);
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:79:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'const char*'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:80:12: note: size_t Print::println(char) <near match>
     size_t println(char);
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:80:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'char'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:81:12: note: size_t Print::println(unsigned char, int) <near match>
     size_t println(unsigned char, int = DEC);
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:81:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'unsigned char'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:82:12: note: size_t Print::println(int, int) <near match>
     size_t println(int, int = DEC);
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:82:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'int'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:83:12: note: size_t Print::println(unsigned int, int) <near match>
     size_t println(unsigned int, int = DEC);
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:83:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'unsigned int'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:84:12: note: size_t Print::println(long int, int) <near match>
     size_t println(long, int = DEC);
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:84:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'long int'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:85:12: note: size_t Print::println(long unsigned int, int) <near match>
     size_t println(unsigned long, int = DEC);
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:85:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'long unsigned int'
ask_receiver:54: error: call of overloaded 'write(uint8_t [32])' is ambiguous
     Serial.write(privateKey);
                            ^
/tmp/arduino_modified_sketch_961866/ask_receiver.pde:54:28: note: candidates are:
In file included from /ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Arduino.h:232:0,
                 from sketch/ask_receiver.pde.cpp:1:
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:129:20: note: virtual size_t HardwareSerial::write(uint8_t) <near match>
     virtual size_t write(uint8_t);
                    ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:129:20: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'uint8_t {aka unsigned char}'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:130:19: note: size_t HardwareSerial::write(long unsigned int) <near match>
     inline size_t write(unsigned long n) { return write((uint8_t)n); }
                   ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:130:19: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'long unsigned int'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:131:19: note: size_t HardwareSerial::write(long int) <near match>
     inline size_t write(long n) { return write((uint8_t)n); }
                   ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:131:19: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'long int'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:132:19: note: size_t HardwareSerial::write(unsigned int) <near match>
     inline size_t write(unsigned int n) { return write((uint8_t)n); }
                   ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:132:19: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'unsigned int'
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:133:19: note: size_t HardwareSerial::write(int) <near match>
     inline size_t write(int n) { return write((uint8_t)n); }
                   ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:133:19: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'int'
In file included from /ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Stream.h:26:0,
                 from /ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/HardwareSerial.h:29,
                 from /ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Arduino.h:232,
                 from sketch/ask_receiver.pde.cpp:1:
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:52:12: note: size_t Print::write(const char*) <near match>
     size_t write(const char *str) {
            ^
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Print.h:52:12: note:   no known conversion for argument 1 from 'uint8_t [32] {aka unsigned char [32]}' to 'const char*'
/tmp/arduino_modified_sketch_961866/ask_receiver.pde: In function 'void loop()':
/tmp/arduino_modified_sketch_961866/ask_receiver.pde:80:51: warning: invalid conversion from 'uint8_t* {aka unsigned char*}' to 'const char*' [-fpermissive]
   driver.printBuffer("Decrypted:", buf, strlen(buf));   
                                                   ^
In file included from /ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/arduino/avr/cores/arduino/Arduino.h:25:0,
                 from sketch/ask_receiver.pde.cpp:1:
/ogobase/home/henk/Downloads/arduino-1.8.5-linux64/arduino-1.8.5/hardware/tools/avr/avr/include/string.h:399:15: note: initializing argument 1 of 'size_t strlen(const char*)'
 extern size_t strlen(const char *) __ATTR_PURE__;
               ^
exit status 1
call of overloaded 'println(uint8_t [32])' is ambiguous

I have also tried:

Serial.write((int)privateKey);

But it gives me gibberish output.

I also tried:

String stringOne = String(privateKey, HEX)
Serial.println(stringOne);

But it gives me the following output:

call of overloaded 'String(uint8_t [32], int)' is ambiguous

Preferably, I'd output it in HEX or some other value which I can transfer over another medium.

Answer 1

Solved it by using a simple for loop

uint8_t i;

for (i = 0; i < (uint8_t)(sizeof(privateKey)); i++) {
if (privateKey[i] < 0x10) { /* To print "0F", not "F" */
  Serial.write('0');
}
Serial.print(privateKey[i], HEX);
}